[media] v4l: Define a pixel format for the R-Car VSP1 1-D histogram engine

The format is used on the R-Car VSP1 video queues that carry
1-D histogram statistics data.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>